Linear Collisions

1. The problem statement, all variables and given/known data
A block of mass m1=2.0 kg slides along a frictionless table with a speed of 10 m/s. Directly in front of it, and moving in the same direction, is a block of mass m2=5.0 kg moving at 3 m/s. A massless spring with spring constant k = 1120 nt/m is attached to the backside of m2 as shown in the diagram below. When the blocks collide, what is the maximum compression of the spring? Assume that the spring doesn't bend and always obeys Hooke's law.

2. Relevant equations
I'm not really sure how to do this but:
p=mv
KE=1/2mv^2
KE=1/2kx^2

3. The attempt at a solution
p1=20kgm/s p2=15kgm/s
KE1=100J KE2=22.5J
I have no idea what to do now though, or even if I need any of that information

mfb's centre of mass approach is good. Alternatively, consider what the relationship is between the velocities when the spring is at maximum compression.
What conservation laws do you know that might be useful?